Freeze-Drying and Wood Shrinkage

The purpose of this research was to assess the effects of vacuum drying at temperatures below 0?C and prefreezing before drying on wood shrinkage. These results were compared with those from air-drying, kiln-drying at 39?C, and final vacuum drying of matched specimens. The tests were conducted on wafers of sapwood of Douglas-fir and Sitka spruce 3 millimeters or less along the grain. Vacuum drying at 5 to 10 microns and at temperatures from -8? to -20?C produced nearly the same shrinkage as did kiln-drying followed by vacuum drying or ovendrying. At the equilibrium weight in freeze drying, however, the residual moisture in the wood ranged from 2.8 to 4.8 percent MC depending upon the particular experiment. When the freeze-dried wafers were dried further in an oven at 105?C or in vacuum at 65?C, additional shrinkage occurred. The combined effect gave shrinkage values which were always greater than those obtained from air-drying at 39?C or vacuum drying at 65?C. The importance of stress relaxation reported by others seems to have relevance to these studies. Prefreezing prior to either air-drying or to freeze drying produced very little reduction in shrinkage in either case. The reduction was about 3 percent on the average. Radial checking and wavy radial edges due to different springwood and summerwood shrinkage rates occurred to about the same extent in the freeze-dried wood as in wood dried in air at 20-39?C. The results support the concept that most, if not all, of the bound water acts as a liquid during drying at to -20?C and that shrinkage accompanies the water loss.

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